Organic electronics, such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), organic photovoltaic cells (OPVs), organic light-emitting electro-chemical cells (OLECs), photo detectors, and all-polymer integrated electronic circuits, have drawn much attention in the past two decades, because they are versatile in material design, light-weight, and suitable for large area application. These organic electronic devices usually comprise a multilayer structure. Hereby it is crucial to deposit the active organic materials on the nano- to mesoscopic scale on a substrate. In addition, the formation of organic multilayer structures has to occur without affecting the underlying layers in an uncontrolled way. The development of cost effective technologies for the mass production of organic electronics will have a decisive role for their competitiveness as compared to current technologies employed. Cost reduction for the mass production is therefore a mandatory prerequisite for the commercial success of organic electronics. In order to achieve this goal solution based deposition methods could provide a solution. However, current technologies do not offer a competitive way to adequately prepare multilayered structures for organic electronics, particularly if they comprise small molecules. This is due to the fact that layers that have already been deposited may be, at least in part, dissolved or washed away when another layer is deposited on it.
In the case of OLEDs, the formation of patterned light emitting layers is an important but difficult step in the production of electroluminescent devices. For example, the formation of separate red, green and blue patterned emitter layers is typically required in the production of electroluminescent full color display devices. Vacuum evaporation (e.g., using a shadow mask) is the most common technique to form each of the patterned layers. However, vacuum evaporation techniques have a couple of drawbacks which may significantly inhibit the commercial development of multilayer structures comprising organic materials such as OLEDs. These techniques are rather complex in terms of equipment needed. In particular for large format displays, other methods for manufacturing patterned layers are needed. Methods based on depositing materials from solution are especially desirable for their expected compatibility with large scale device fabrication.
The possible technologies are, for example, ink-jet printing, dip coating, spin coating, letter-press printing, screen printing, doctor blade coating, and slot-die coating etc. Ink-jet printing is particularly preferred as it allows high resolution displays to be prepared.
The prior art provides compositions being useful in order to process low molecular weight organic light emitting and charge transporting materials. However, it is a permanent desire to improve the performance of the OLED layer, such as efficiency, lifetime and sensitivity regarding oxidation or water.
Currently soluble OLED inks are prepared and processed in multiple layers, initially PEDOT is applied and then dried and baked to remove alt water. The next layer to be printed is the HTL (Hole transport layer) this is dried and baked (optionally crosslinking).
Finally the EML (Emission Material Layer) is applied and baked. The HTL is frequently a crosslinkable material this is to ensure that there is no intermixing of the HTL and EML layers. This approach makes it difficult to be able to change the HTL dependant on the colour. As a singlet blue is frequently used then this ideally necessitates a different HT layer in order to maximise the lifetime. Therefore, at present the process using conventional inks is very complicated and expensive.
In addition thereto, the formulation should enable a low-cost and easy printing process. The printing process should allow a high quality printing at high speed.
It is therefore desirable to have improved formulations comprising an OSC that are suitable for the preparation of OE devices, especially thin film transistors, diodes, OLED displays and OPV cells, which allow the manufacture of high efficient OE devices having a high performance, a long lifetime and a low sensitivity against water or oxidation. One aim of the present invention is to provide such improved formulations. Another aim is to provide improved methods of preparing an OE device from such formulations. Another aim is to provide improved OE devices obtained from such formulations and methods. Further aims are immediately evident to the person skilled in the art from the following description.